Achieving excellent strength-ductility combination in Co60.8-xNi39.2Alx eutectic medium-entropy alloys

Abstract

In this study, we design and prepare three eutectic medium-entropy alloys (EMEAs) with the composition of Co60.8-xNi39.2Alx (x = 19, 21.6 and 24). All three alloys exhibit a FCC-B2 dual-phase structure, which evolves from hypoeutectic to eutectic and then to hypereutectic with increasing Al content. The Co39.2Ni39.2Al21.6 EMEA demonstrates a complete lamellar eutectic structure with fine and disperse ordered L12 precipitations in the FCC phase, while the nano-scale strip L10 martensite is present in the B2 phase. The Co39.2Ni39.2Al21.6 EMEA exhibits an exceptional combination of strength and ductility, with high yield strength and elongation that are superior to other FCC-B2 dual-phase EMEA reported in the past five years. The superior ductility was related to the stable K–S interface orientation during the deformation process and the transformation-induced plasticity effect in B2 phases. The high strength is derived from the interface strengthening effect of the FCC-B2 interfaces. Moreover, the hindering of dislocation slipping by the ordered L12 precipitates and the Lomer-Cottrell locks in the FCC lamellar can also enhance the strength of the alloy. These findings have important implications for the structural design of eutectic high/medium-entropy alloys.